The KamLAND-Zen 800 experiment is searching for the neutrinoless double-beta\ndecay of $^{136}$Xe by using $^{136}$Xe-loaded liquid scintillator. The liquid\nscintillator is enclosed inside a balloon made of thin, transparent,\nlow-radioactivity film that we call Inner Balloon (IB). The IB, apart from\nguaranteeing the liquid containment, also allows to minimize the background\nfrom cosmogenic muon-spallation products and $^{8}$B solar neutrinos. Indeed\nthese events could contribute to the total counts in the region of interest\naround the Q-value of the double-beta decay of $^{136}$Xe. In this paper, we\npresent an overview of the IB and describe the various steps of its\ncommissioning minimizing the radioactive contaminations, from the material\nselection, to the fabrication of the balloon and its installation inside the\nKamLAND detector. Finally, we show the impact of the IB on the KamLAND\nbackground as measured by the KamLAND detector itself.